It’s a prom must-have, right up there with the dress and shoes: The guy’s tie must match the girl’s gown. And many teens today use cellphones to aid the color coordination.

Kourtney Ziercher took a picture of her dress in the store to send to her date, Michael George, for her prom last year at Barat Academy in Chesterfield, Mo.

“I told him it was burnt orange, and he got the tie to match,” she said.

“She knew that it was not a typical color, but the tie I got even had a little design on it that matched the design on the dress,” Michael said, referring to a light tiger stripe print on the fabric. “She was really excited about it. The girl’s dress is a big deal. If the outfit doesn’t go perfectly, if the guy isn’t matching, it’s a problem. I made sure it worked.”

In Fort Collins, Amy Weintraub sent a picture of her chocolate- brown dress to her date, Luke Siddens, and he used the image to get a matching tie and vest for their prom last week at Poudre High School.

“I didn’t realize he wanted to match me,” Amy said. “He just kind of decided that on his own. It was a little lighter than the dress, but it matched pretty well.”

For homemade dresses, ties can always be made from leftover fabric. For a 2010 prom at the Stanley Humphries Secondary School in Castlegar, British Columbia, Arielle Roberts made her own dress with the help of a grandmother and aunt. Her date’s mom sewed a matching tie “when the green bow-tie and cummerbund she’d ordered for him weren’t quite the right shade,” Arielle said.

But with so many dresses store-bought these days, and with cellphone photos sometimes producing unreliable hues, stores often provide physical swatches to assist in color coordination.

Sarah LaRue, a senior at Sheldon High School in Eugene, Ore., picked out a dress at the local David’s Bridal and was happy to have the fabric scrap to give her date, which she was able to buy for a $1 with her dress purchase. “It’s a turquoise blue, and with that dress, you need the exact color to match it,” she said. “If it was even a little bit off, it wouldn’t look right.”

David’s Bridal also has a partnership with Men’s Wearhouse that makes it easy for young men to get accessories in coordinated hues for their dates’ dresses. Guys can order ties, vests and cummerbunds from Men’s Wearhouse using the same terms David’s Bridal uses to identify the dress colors — names like “watermelon pink” or “Malibu blue” — and be guaranteed an ensemble that works.

Many couples also plan matching corsages and boutonnieres. “The girls are saying, ‘This is what I’m wearing and I want it to match the flowers,’ ” said Pennylyn Kaine, owner of Blossom & Bee Floral and Event Design in Newfoundland, N.J.